The present invention relates to target structures that are used to provide sputtering material for sputter coating sources, and particularly to such sources which are made up of a composite of materials and the techniques for their fabrication.
Sputtering, is a process that falls in the general class of vacuum coating processes, which are used to deposit a thin layer of a desirable material on the surface or surfaces of another object to provide a particular function or to enhance the appearance of the object. As an example, thin films of aluminum or aluminum alloys, on the order of a micrometer in thickness, are applied to silicon wafers in the production of integrated circuits to form the electrical connections between individual semiconductor devices. As another example, thin films of aluminum, chrome, brass, and other metals are applied to the surfaces of plastic objects, such as automotive headlamp bezels, door handles, and door lock plungers, to give those objects a pleasing metallic appearance.
The sputtering process is a plasma process by which positive ions of an inert gas impinge upon the surface of a target material, which is the material used to form the desired film. As an example, in a sputtering process of coating a plastic object with a thin film of chromium alloy, the target material would be the chromium alloy. Sputtering is accomplished from a device called a sputter coating source. Such a source embodies electrical means for biasing a target material structure with a negative voltage, either DC for conductive targets, or RF for non-conductive targets, so the target will attract positive ions from a plasma of an inert gas that is established in the region of the target. The sputter coating source also contains means for cooling the target structure and often magnetic means for containing and enhancing the plasma.
Positive ions extracted from the plasma are accelerated to a high kinetic energy and strike the surface of the target structure, where part of the kinetic energy is changed to heat, and part is imparted by momentum transfer to atoms of the target material. Atoms that gain sufficient energy to overcome their binding energy escape from the target surface and are ejected into the vacuum chamber that houses the process. Objects placed in line-of-sight of an operating source are coated by the atoms ejected from the target surface.
Because target material is used up in the sputtering process, sputter coating sources are designed and built so that used targets can be removed and new targets mounted at intervals. The mounting of a new target structure is called replenishment.
There are many kinds and shapes of sputtering sources, and therefore many sizes and shapes of sputtering target structures, including round disks, conical shapes, and rectangular shapes. Regardless of the design and shape of a sputter coating source and the resulting profile of a demountable target structure, all have a problem: not all of the target material is depleted before the target must be changed to avoid erosion of the source itself. The requirements for providing a self-sustaining plasma and the way ions are attracted to the target surface are such that a perfectly even erosion over a target surface cannot be maintained in practice. The inevitable result is that erosion is preferential in a particular pattern that is a function of the source design. This effect is true for sources known as magnetically enhanced diode sputtering sources, hereinafter, magnetrons, in which a magnetic field is used to enhance and maintain the plasma. Whereas in triode sources and in diode sources using a cylindrical hollow cathode, this preferential pattern of erosion is not a significant practical problem.
FIGS. 1A, 1B and 1C illustrate an example of the prior art corresponding to a one-piece target structure for a diode sputter coating source known as a rectangular planar magnetron. A rectangular target structure 11 has a surface 12 which, during deposition, is presented to the plasma in conjunction with the sputter coating source. The surface is typically flat and of uniform thickness before sputtering erosion begins. The confinement of the plasma by the magnetic field of the sputter coating source is such that erosion is preferential in an area 13 of the surface presented to the plasma. As erosion proceeds most material is eroded from area 13. When erosion has proceeded nearly through the target structure, so that further erosion might damage the sputter coating source to which the target is attached, the erosion profile is typically as shown in the two section views FIG. 1B and FIG. 1C, as profile surface 14. At that point in the operation, the target structure can no longer be used and must be removed from the sputter coating source and replaced by a new target structure. The cross-hatched portions 15 of FIG. 1B and FIG. 1C indicate target material that is not used in the sputtering operation. The unshaded area 16 represents material that has been sputtered away to form a substrate film, although not all of this material reaches the substrate. The ratio of sputtered material to unused material in the target structure is seldom greater than 40 percent for a rectangular planar magnetron sputter coating source.
FIGS. 2A and 2B show a plan view and cross-sectional view, respectively, of a prior art target structure for a round planar magnetron which includes a means of mechanically manipulating the magnets to increase the erosion area. The target structure is labelled 21, the uneroded original surface is labelled 22, the preferential erosion area is labelled 23, the erosion profile at end-of-life is labelled 24, the unused volume is labelled 25, and the sputtered volume is labelled 26. In this type of round magnetron source, the ratio of used to unused material is higher than for a rectangular planar magnetron, but is seldom more than 60 percent.
FIGS. 3A and 3B show a plan view and a cross-sectional view, respectively, of a prior art target structure for a typical conical magnetron sputter coating source. The target structure is labelled 31, the uneroded original surface is labelled 32, the preferential erosion area is labelled 33, the erosion profile at end-of-life is labelled 34, the unused volume is labelled 35, and the sputtered volume is labelled 36. With this type of source the ratio of used to unused material may be as high as 70 percent.
Of primary interest for the purpose of the present invention is the fact that the end-of-life profile for a sputtering target is a function of the sputter coating source, and is a known quantity which can be determined emperically with a very small margin of error for each type of source. As a result, in a production process with a particular sputtering machine and using a particular sputter coating source, the used targets have the same profile, within small limits of deviation.
In the general art of sputtering target fabrication, there are several methods of forming the target material, and the method used is generally chosen as a result of the physical properties of the material. For metals with relatively low melting points, and mixtures of these that form alloys, the most often used method is vacuum melting. The materials are mixed, melted together, and poured into a mold in a vacuum furnace, which helps in outgassing volatile constituents of the mix. After vacuum casting and cooling, the resulting ingot, often weighing several hundred pounds, is sliced and machined to the desired final shapes to be joined to a particular sputter coating source. Vacuum melting helps to provide purity and homogeneous alloys, and results in materials that do not outgas into the vacuum chamber as sputtering proceeds. One disadvantage of this method is that it is not energy efficient. Also, a considerable amount of expensive machining must be employed to provide the final target profile.
Materials with high melting points, and mixtures of materials with one or more of the constituents having high melting points, such as the refractory metals, are formed by the arts of powder metallurgy. The constituents are reduced to a powder form, then the powders are mixed and placed in a mold. Applications of pressure and temperature are used to compact the powders to the point that the powders adhere and form a single, solid structure. The compacted shape is then machined to the final dimensions to be joined to a particular sputter coating source. The methods of powder metallurgy have the advantage of fabrication to near net shape, avoiding much of the machining requirements associated with vacuum melting techniques. There have been disadvantages to powder metallurgy targets in the past, the most critical being the inability of the target manufacturers to achieve high density materials. Gasses trapped in low density materials are released during sputtering and contaminate the sputtering process. Also, low density materials do not have the strength of vacuum melted materials, and do not stand up well to thermal stresses and clamping requirements in the sputtering process.
There are other methods sometimes used to form target materials, such as chemical vapor deposition from hydro-carbon gasses to form high purity carbon and graphite, and chemical techniques for targets that are chemical compounds, such as metal oxides.
Regardless of the method used to form the target material, the material must finally be formed to a final shape, such as a disk, a cone, a rectangle, or a square, depending on which profile is required for the particular sputter coating source to be used. Some materials used in sputter coating processes are very difficult to machine, others are fragile, and others are porous. Also the target requirement is sometimes for a profile that is larger in diameter or in length and breadth than can be provided in a single piece by the fabrication method used. It is therefore a common practice to fabricate targets with a backing plate of copper or some other material with good electrical and thermal conductivity, and the desired sputtering material is bonded to the backing plate. In the case of fragile materials, such as carbon and ceramic materials, a backing plate is nearly always used.
The two general methods of bonding to backing plates are by soldering techniques employing low melting point materials, such as indium, lead, and tin, and by adhesive bonding with usually thermosetting adhesives, such as epoxies, which are filled with copper, aluminum, or other powders to provide heat and electrical conductivity.
There are disadvantages to both of these bonding techniques. The solder techniques limit the power density that can be applied to a target to a point that the target temperature will not exceed the melting point of the solder material. The adhesive technique also limits the power density applicable because of the generally poor heat conductivity of the filled thermosetting adhesive.
Given the above-described disadvantages of standard target shapes in achieving full use of the target material and the disadvantages of present bonding techniques of the target to the backing plate, what is needed is a new target structure which can achieve as nearly as practicable, complete use of the expensive target materials and a method of joining that target structure to a suitable backing plate.